482 A. M. Banta 



be little responsive to this influence. It is a part of the adaptive 

 economy of nature, a frugality long noted with reference to eye 

 structures, but apparently less remarked with regard to light per- 

 ception in general. 



Such animals as the cockroach and Oniscus (cf. Cole, '07, pp. 

 373-380) remain in relatively closely circumscribed and restricted 

 dark situations by day. While their habits are such that highly 

 discriminative eyes would be of little use to them, slightly acute 

 organs for discriminating differences in intensity serve them very 

 well, aiding them in reaching and remaining concealed within 

 their diurnal haunts. Asellus often lives in fairly exposed situa- 

 tions, but, like Oniscus, it tends to seek the darker of the avail- 

 able regions. It possesses better powers of discrimination than 

 Cascidotea, and this serves it well in aiding it to find quite small 

 and restricted shaded areas, which Caecidotea can not do. 



The experiments discussed in this paper clearly show that the 

 subterranean species, Caecidotea stygia, has greater sensitiveness 

 to mechanical stimulation (whether purely tactile or vibrations) 

 than its near relative, Asellus communis, which lives above 

 ground. Caecidotea proved decidedly the more sensitive, both 

 in having a lower threshold of stimulation and in responding 

 more generally and more vigorously to such stimuli. Hence these 

 isopods furnish an undoubted case in support of the common 

 belief that cave animals have acquired greater sensitiveness to 

 mechanical stimulation than their near relatives living in other 

 situations. 



Since Caecidotea was shown to be much less sensitive to light 

 than Asellus, its greater sensitiveness to mechanical stimulation 

 is a good illustration of the principle of increase in sensitiveness to 

 one sort of stimulation in compensation for the partial loss of 

 sensitiveness to another, i.e., the organism is so adapted to its 

 environment that when one influence ceases to regulate its move- 

 ments another acquires increased importance and in a measure 

 replaces it. 



There still remains one point to be considered — the bearing of 

 these experiments on a possible explanation of the occurrence of 

 the cave species within caves rather than outside of them and the 



